In the English language, some seeming non-sequiturs are used to indicate that someone suspects he has been or is being tricked. I've known these vernacular phrases to confuse those who speak English as a second language. Some of the more common examples are:
"I don't buy it."
Referring to the fact that if the doubted statement were a product, the doubtor would not spend his hard-earned money on it.
"It's too quiet."
A movie cliche. Indicates the protagonist suspects an ambush or trap. The phrase generally precipitates loudness in the form of a fight or chase sequence.
"My left foot"
I can't even offer conjecture about how this one arose. It has since evolved into the emphatically vulgar "My ass". This in turn has spawned the more embellished, "My fat ass".
"Something is rotten in the state of Denmark."
A reference to Shakespeare's Hamlet, a play about Denmark and the large amounts of betrayal involved in it's politics. Indicates that the if the doubtor happened to be in Denmark, the heir to the throne might be taking power quite soon. But he isn't so something else suspicious must be occuring.
"Something smells fishy."
Basically a statement to the effect that the offered 'veracity' is corrupted and emitting an odor in much the same way as corrupted fish.
"This doesn't add up."
A term that is possibly related to formal logic (?) in that a suspect explanation is missing a few vital points without which the sum of the parts do not add up to the whole of the outcome.
The doubtor professes to agree with the doubtee, but uses sarcasm to indicate that he is only being as sincere as he suspects the doubtee of being. Which is to say, not very sincere.